In recent years, along with the spread of color images, demands for higher image quality of the color image have increased. Regarding a digital full-color copier or printer, a color image document is subjected to color separation with a color filter of each of blue, green, and red and, thereafter, a latent image corresponding to the original image is developed by using a color toner of each of yellow, magenta, cyan, and black. Therefore, the coloring power of a colorant in each color toner exerts a significant influence on the image quality. However, in general, regarding dispersion of a pigment into various media, it is difficult to make the pigment fine sufficiently and disperse the pigment uniformly.
Examples of yellow pigments having high transparency, a high coloring power, and excellent weatherability include pigments having an isoindoline skeleton, e.g., C. I. Pigment Yellow 185. However, regarding such a pigment, autoagglutination occurs easily, and a sufficient dispersion state is not obtained. Consequently, there is a problem in that a performance intrinsic to the pigment with respect to the transparency and the saturation is not delivered sufficiently.
In order to solve the above-described problem, PTL 1 describes a pigment dispersion liquid containing a pigment derivative in which a sulfonic acid group is introduced into an isoindoline based pigment. However, the dispersibility of the pigment still has room for improvement.